Published data were used to analyse the long-term mean sea level trends at six locations located in the Indian Ocean. Stations were located in South Africa, Yemen, India, Thailand and Australia.
The data used were all from the website of PSMSL (the Permanent Service for Mean Sea Level) which maintains a huge, worldwide database of sea level data. Only raw data were used (ie as originally recorded, without any adjustments). These data are relative to the local land levels.
The span of records of the data ranged from 58 years to 142 years; the earliest record (at Mumbai) commencing in 1878. Apart from at Fremantle all of the stations had badly broken (incomplete) records. However, the considerable span of the records ensured that some indications of the long-term trends were possible.
The plots shown below are of the annual means at each of the six locations. On each plot a linear “best fit” trendline has been added. The “slope” of the trendline, expressed in cm / century, are included in the table above.
ANNUAL MEAN SEA LEVEL TRENDS FOR INDIVIDUAL STATIONS
Simons Bay (South Africa) 1958-2015
Aden (Yemen) 1880-2012
Mumbai / Bombay (India) 1878-2010
Chennai / Madras (India) 1916-2011
Ko Taphao Noi (Thailand) 1940-2012
Fremantle (Australia) 1897-2015
DISCUSSION AND COMMENTS
- Five of the six stations had badly broken (incomplete) records but long-term trends are still able to be identified.
- The annual mean sea level trend-lines for all six stations are remarkably linear with no indication of increasing or decreasing trends in recent decades.
- The values of the trendline slopes (which do not allow for changes in local land levels) are less important than the changes (acceleration or decceleration) in the trendline slopes.
- There is a wide range in the slopes of the trendlines (+5.7 to +22.4 cm/century) at different locations, the smaller values occurring at the four tropical locations.
This article was written by Brian Gunter of Narooma, NSW. In his previous life Brian was an engineering hydrologist involved over many years in the analysis of rainfall and river flow data for the planning of water resources projects in Australia, Asia and Africa. In recent years he has been one of the Marine Rescue NSW (previously Royal Volunteer Coastal Patrol) volunteer weather observers who operate the Narooma station for the Bureau of Meteorology.